I don't understand, this happened years ago. Both Rocket League and Fall Guys originally came out on Steam, but disappeared from the Steam Store and became Epic Games exclusives. If you still own a Steam copy though, you can still play it.

Is this saying that the game will be unplayable without an Epic Games authentication or something? It's not like the Steam versions are getting updates anymore, so they should still be playable with other Steam members. Unless the publishers are pushing an update to the Steam version as well.

EDIT: Upon rereading, I see these games are being ported into another game. Does this affect Steam users at all, since Fortnite is an Epic Games exclusive?

I won't touch Epic Games, so this doesn't seem to have any effect on me.